The present invention relates generally to medical devices and particularly to a stent with a pushing member at the proximal or distal end of the stent.
The use of stents to treat various organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like, has become common in recent years. Stents are most commonly used to treat blockages, occlusions, narrowing ailments and other similar problems that restrict flow through a passageway. One area where stents are now commonly used for treatment involves implanting an endovascular stent into the vascular system in order to improve or maintain blood flow through narrowed arteries. Stents have been shown to be useful in treating various vessels throughout the vascular system, including both coronary vessels and peripheral vessels (e.g., carotid, brachial, renal, iliac and femoral).
The use of stents in coronary vessels has drawn particular attention from the medical community because of the growing number of people each year that suffer from heart problems associated with stenosis (i.e., narrowing of a vessel). This has led to an increased demand for medical procedures to treat such problems. The widespread frequency of heart problems may be due to a number of societal changes, including the tendency of people to exercise less and the prevalence of unhealthy diets, in conjunction with the fact that people generally have longer life spans now than previous generations. Stents have become a popular alternative for treating coronary stenosis because stenting procedures are considerably less invasive than conventional procedures. Traditionally, stenosis of the coronary arteries has been treated with bypass surgery. In general, bypass surgery involves splitting the chest bone to open the chest cavity and grafting a replacement vessel onto the heart to bypass the blocked, or stenosed, artery. However, coronary bypass surgery is a very invasive procedure that is risky and requires a long recovery time for the patient. To address the growing demand for non-invasive medical procedures for the treatment of coronary vessels and other passageway problems, the medical community has begun to turn away from conventional invasive procedures like bypass surgery and increasingly the treatment of choice now involves various types of stenting procedures.
Many different types of stents and stenting procedures are possible. In general, however, stents are typically designed as tubular support structures that may be inserted percutaneously and transluminally through a body passageway. Traditionally, stents are made from a metal or other synthetic material with a series of radial openings extending through the support structure of the stent to facilitate compression and expansion of the stent. Although stents may be made from many types of materials, including non-metallic materials, common examples of metallic materials that may be used to make stents include stainless steel, nitinol, cobalt-chrome alloys, amorphous metals, tantalum, platinum, gold and titanium. Typically, stents are implanted within a passageway by positioning the stent within the area to be treated and then expanding the stent from a compressed diameter to an expanded diameter. The ability of the stent to expand from a compressed diameter makes it possible to thread the stent to the area to be treated through various narrow body passageways while the stent is in the compressed diameter. Once the stent has been positioned and expanded at the area to be treated, the tubular support structure of the stent contacts and radially supports the inner wall of the passageway. As a result, the implanted stent mechanically prevents the passageway from closing and keeps the passageway open to facilitate fluid flow through the passageway.
Stents can generally be characterized as either balloon-expandable or self-expandable. However, stent designs and implantation procedures vary widely. For example, although physicians often prefer particular types of stents for certain types of procedures, the uses for balloon-expandable and self-expandable stents frequently overlap and procedures related to one type of stent are frequently adapted to other types of stents.
Balloon-expandable stents are generally used to treat stenosis of the coronary arteries. Usually, balloon-expandable stents are made from ductile materials that plastically deform relatively easily. In the case of stents made from metal, 316L stainless steel that has been annealed is a common choice for this type of stent. One procedure for implanting balloon-expandable stents involves mounting the stent circumferentially on the balloon of a balloon-tipped catheter and threading the catheter through a vessel passageway to the area to be treated. Once the balloon is positioned at the narrowed portion of the vessel to be treated, the balloon is expanded by pumping saline through the catheter to the balloon. The balloon then simultaneously dilates the vessel and radially expands the stent within the dilated portion. The balloon is then deflated and the balloon-tipped catheter is retracted from the passageway. This leaves the expanded stent permanently implanted at the desired location. Ductile metal lends itself to this type of stent since the stent may be compressed by plastic deformation to a small diameter when mounted onto the balloon. When the balloon is then expanded in the vessel, the stent is once again plastically deformed to a larger diameter to provide the desired radial support structure. Traditionally, balloon-expandable stents have been more commonly used in coronary vessels than in peripheral vessels because of the deformable nature of these stents. One reason for this is that peripheral vessels tend to experience frequent traumas from external sources (e.g., impacts to a person's arms, legs, etc.) which are transmitted through the body's tissues to the vessel. In the case of peripheral vessels, there is an increased risk that an external trauma could cause a balloon-expandable stent to once again plastically deform in unexpected ways with potentially severe and/or catastrophic results. In the case of coronary vessels, however, this risk is minimal since coronary vessels rarely experience traumas transmitted from external sources.
Self-expandable stents are increasingly used and accepted by physicians for treating a variety of ailments. Self-expandable stents are usually made of shape memory materials or materials that act like a spring. Typical metals used in this type of stent include nitinol and 304 stainless steel. A common procedure for implanting a self-expandable stent involves a two-step process. First, the narrowed vessel portion to be treated is dilated with a balloon as described above. Second, the stent is implanted into the dilated vessel portion. To facilitate stent implantation, the stent is installed on the end of a catheter in a compressed, small diameter state and is usually retained in the small diameter by inserting the stent into a sheath at the end of the catheter. The stent is then guided to the balloon-dilated portion and is released from the catheter by pulling the retaining sheath off the stent. Once released from the retaining sheath, the stent radially springs outward to an expanded diameter until the stent contacts and presses against the vessel wall. Traditionally, self-expandable stents have been more commonly used in peripheral vessels than in coronary vessels due to the shape memory characteristic of the metals that are used in these stents. One advantage of self-expandable stents for peripheral vessels is that traumas from external sources do not permanently deform the stent. Instead, the stent may temporarily deform during an unusually harsh trauma but will spring back to its expanded state once the trauma is relieved. Self-expandable stents, however, are often considered to be less preferred for coronary vessels as compared to balloon-expandable stents. One reason for this is that balloon-expandable stents can be precisely sized to a particular vessel diameter and shape since the ductile metal that is used can be plastically deformed to a desired size and shape. In contrast, self-expandable stents are designed with a particular expansible range. Thus, after being implanted, self-expandable stents continue to exert pressure against the vessel wall.
Typically, stents are provided with markers and/or pushing members that are attached or formed along the proximal and/or distal ends of the stent structure. These features may be used for a number of purposes and usually serve more than one function. For example, markers are usually provided at both the proximal and distal ends of the stent to assist the physician in positioning the stent during stenting procedures. Generally, separate markers are needed on most stents since the stent structure itself cannot usually be seen easily on x-ray and other visualization equipment. This is due in part to the types of material that are usually used in stent structures and the slenderness of the structural members in the stent structure. Markers address this visualization problem by providing features with increased radiopacity along the proximal and distal ends of the stent. The features (i.e., the markers) are typically larger in width than the structural members of the stent structure and usually are filled with a radiopaque material like gold or platinum. As a result, the radiopaque material in the markers can be seen more easily on the physician's visualization equipment than the stent structure itself.
Pushing members are also used at the proximal and/or distal ends of many stents. Pushing members are particularly useful for self-expandable stents but may also be used on balloon-expandable stents. In either case, the pushing members provide a separate contact surface at the end of the stent that may be pushed against. As a result, the stent structure itself is not directly pushed against. In the case of self-expandable stents, the pushing members of the stent are used at several different times. For example, during the manufacture of self-expandable stents and their corresponding delivery systems, the stent must be loaded into the delivery system in a compressed state. Delivery systems for self-expandable stents are well known to those in the art, and therefore, a detailed description is not necessary. However, as described above, delivery systems for self-expandable stents usually include a retaining sleeve at the end of a catheter which restrains the outer surface of the stent and keeps the stent compressed until the stent is released at the site of implantation. A common manufacturing method for loading stents into the retaining sleeve involves compressing the stent while at the same time pushing on one end of the stent in order to slide the stent into the sleeve. Alternatively, the stent may be compressed and pushed into a transfer tube first and then pushed again through the transfer tube into the delivery system.
Pushing members are also used on the proximal end of self-expandable stents in order to release the stent from the delivery system for implantation. As previously described, self-expandable stents are released for implantation by pulling the retaining sleeve off the stent. Typically, the delivery system also includes a holder within the retaining sleeve which contacts the proximal end of the stent. Generally, the holder and the sleeve are designed to move relative to each other so that as the sleeve is pulled back, the holder can be maintained in place. As a result, the holder prevents the stent from moving rearward with the retaining sleeve as the sleeve is pulled back. In effect, the stent is pushed out of the sleeve by the holder as the sleeve is pulled rearward.
Typically, the markers on a stent are also used as pushing members and vice versa. One problem with current stent structures is that the pushing force that is transmitted by the pushing member to the stent structure is concentrated onto a small number of structural members in the stent structure. As a result, the pushing force can cause the structural members to bend and deform as the stent is being pushed. In extreme cases, this concentrated force can permanently deform parts of the stent structure. This problem is of particular concern on longer length stents. Generally, most stents that are currently used for medical treatments are 8 cm or less in length. However, stents that are longer than 8 cm are becoming more common to treat various peripheral arteries, such as the superficial femoral artery. When longer stents like these are pushed, either during loading into the delivery system or during release, higher frictional forces must be overcome in order to move the stent. The longer length of some of these stents also makes the stents generally less stable than shorter stents. As a result, the bending and deforming problems that may occur when pushing on a stent tend to be more pronounced and damaging on longer stents. However, these types of problems may exist with all stents. Because most pushing members are also used as markers, it has been difficult to identify solutions that minimize these types of bending problems while also providing an acceptable structural member for visualization. Moreover, the pushing members must not interfere with compression and expansion of the stent, which has also limited the alternatives available.
Accordingly, it is apparent to the inventor that an improved marker and/or pushing member is desired for the proximal and distal ends of a stent. A solution to these and other problems is described more fully below.